This invention relates to switches and more particularly to electrical switches.
In one aspect of this invention, a push-button or key operation is provided which stimulates tactile breakthrough. The term "tactile breakthrough" is applied to the feeling of release or free movement of a mechanical key or push-button imparted to the operator after a predetermined point in the pressing of the key is reached. Generally, this feeling is realized when the operated key or push-button has finished or is certain to finish its task. For example, when a typewriter key is pushed to the point where the alphanumeric holding arm is thrown against the platen, the remaining motion of the key is made with far less force -- hence, tactile breakthrough. The free movement tells the operator that the task has been completed.
Tests made under scientifically controlled conditions, have shown a marked difference in operational performance between those push-buttons which exhibit tactile breakthrough and those that do not. Fewer errors and greater speed of operation are exhibited on tactile breakthrough push-buttons. In these days, more and more electronic push-buttons are being employed to operate such devices as typewriters, calculators, cash registers, telephones, and the like. These push-buttons, rather than manually operated keys having mechanical linkages, are becoming a greater part of industry. It, therefore, becomes increasingly important that such electronic push-buttons operate accurately and at high speed. The need for greater accuracy and speed has necessitated the development of electronic push-buttons which simulate tactile breakthrough.
It is believed that present push-buttons which attempt to simulate tactile breakthrough are of a complicated construction, usually involving a combination of springs or similar means. Such devices are believed to be expensive and have a limited life. The complexities of some of these devices are believed to also result in increased expense of maintenance.
A switch which exhibits a tactile breakthrough characteristic may have, in accordance with the invention discussed hereinafter, any form of overall construction. One type of construction, however, is the switch which operates in response to a magnetic field.
Switches employing a liquid conductor as a throw are well known. Of these switches, there have been proposed a number of switches which seek to distort or displace a liquid conductor to thereby couple electrical contacts and to use the self-restorative forces of such liquid conductors to uncouple the electrical contacts. One such device is proposed by Donald S. Rich (one of the co-inventors herein) in an application for United States Letters Patent for SWITCHES, Ser. No. 345,358, and filed Mar. 27, 1973. Other devices are proposed by Schmid in U.S. Pat. No. 3,358,109, Lanctot in U.S. Pat. No. 3,184,693, and Ubukata et al. in U.S. Pat. No. 3,377,445.
Each of these devices has in common the moving of mercury within confined spaces so as to control its flow, thereby forming a fluid throw. The aforementioned patent by Schmid proposes, in one embodiment, a plate made of insulating material having recesses therein. Within the recesses are the exposed ends of electrical contacts. Droplets of mercury fill the remainder of the recesses. Centrally disposed between the two recesses is a third recess larger than the other two and having a larger pool of mercury therein. The insulating plate is covered with a second insulating plate which encloses each of the recesses and forms a capillary path between each of the recesses communicating with the third recess. The top insulating plate has an aperture with a membrane thereacross by which means pressure may be placed upon the larger droplet of mercury in the third recess. By placing pressure on the third droplet of mercury by depressing the membrane, Schmid proposes that the mercury traveled through the capillaries to join with the other two droplets thereby completing an electrical contact. The capillary channels, however, are, in effect, enclosed tunnels. Schmid teaches no means of venting the gases as the gases flow into the capillary channels. The gases therein must be compressed. Such compressed gases would, it is believed, resist the advance of mercury thereby retarding or defeating the action of the switch.
The device of Schmid also has a further disadvantage. Mercury under heavy handling or vigorous actuation tends to subdivide into tiny droplets which become dispersed into all available voids. In addition, sustained pressure may promote adhesion of the mercury to the walls of the channels, thereby delaying or even preventing the retreating of the mercury upon the removal of the switching pressure. If the mercury is thus inhibited in its motion, the breaking of electrical contact will lag significantly behind the switching action or not take place at all. What is more, the device as taught by Schmid provides that the capillary connecting tubes or channels should have a smooth surface. A smooth surface would work against proper operation. Indeed it is believed that it is well known that a fine roughening of the surface is preferred and often referred to as "alacritizing" by those experienced in the art.
The device disclosed by Lanctot, in his aforementioned patent, like that disclosed by Schmid employs the use of a flexible diaphragm to distort a droplet of mercury. Lanctot's purpose, however, is to fill a gap in a high frequency strip line so that electric current will encounter a minimum discontinuity in passing therethrough. The transmission line characteristics require that the geometry of the liquid conductive path that bridges the gap during switching match closely that of the members it is joining. By depressing the membrane or diaphragm, Lanctot proposes to compress a mercury pool to fill a cavity of a predetermined shape bounded at both ends by the interrupted edges of the strip line. Like Schmid, Lanctot provides no means of venting as the mercury is forced into the sharp rectangular corners formed by the strip line. Lanctot's purpose is to form a coordinated strip line mercury switch rather than a switch, such as that proposed by Schmid, that would be insensitive to the pull of gravity.
Ubukata et al. teaches a time delay switch which is clearly position-sensitive and requires a movable container within a container. The inner container has a hole in the bottom and is disposed in the confined pool of mercury in the outer container. The mercury is permitted to flow into and out of the inner container through a conduit having angularly disposed side walls which permits the mercury to flow unrestricted by the movement of gases within the switch. When the inner container is lifted out of the pool of mercury, it brings with it a portion of the mercury which then flows slowly from the second container back into the first container, thereby forming the time delay. Ubukata et al. device is clearly position-sensitive and is ill suited for a quick response as that proposed herein or suggested in the aforementioned Rich application.
Thus, the prior art devices exhibit certain disadvantages. The flow of mercury or another liquid conductor is achieved only at the expense of either being position-sensitive (Lanctot) or work against gases compressed by the flow of the liquid conductor (Schmid and Lanctot).
The aforementioned Rich application proposes a switch intended to overcome the deficiency of the prior art by providing a position insensitive liquid state switch having a plunger for displacing the liquid from a pool into a restricted passage-way. The passage-way is defined by the plunger and a side wall of the cavity. The surface tension of the liquid is used to inhibit its entrance into the passage-way.
Clearly, the narrower opening from the liquid pool into the passage-way, the more likely it is that the liquid conductor will, due to the forces of its surface tension, be restrained from entering therein. On the other hand, the opening must be large enough to permit the forces exerted upon the plunger to overcome this surface tension so that the liquid will be displaced into the passage-way when it is desired to operate the switch. Converging walls of the plunger and the cavity are employed to encourage the self-propelled removal of the displaced liquid from the passage-way when the plunger is withdrawn from the liquid pool. This requirement provides additional parameters to the design of the switch. The electrical contact in the cavity wall should be as far removed from the liquid pool as possible so that any shock or vibrational forces will not so deform the liquid as to cause it to enter the passage-way and prematurely couple the contacts and "misfire" the switch. Preferably such a contact should also be located at or near the apex of the converging walls so that it is much more difficult for prematurely displaced liquid to reach the contact and couple it to the pool. On the other hand, this position is limited by the lowermost position of the plunger -- since the contact must remain in the passageway. However, the result is that with the plunger fully withdrawn, the contact is now spaced a greater distance from the wall of the plunger and is therefore in a more open or accessible position in the passageway. The resulting spacing has been observed to leave the contact vulnerable to premature coupling under displacement of the liquid due to shock and vibration. Further, it has been observed that the flow of the liquid up into the passageway is in a direction substantially opposed to the downward direction of movement of the plunger displacing the liquid. Thus, by the resolution of forces, it is believed that the net force available by the plunger is thereby reduced. This loss due to the counterforces of the liquid upon the plunger makes the plunger less efficient. The force available is a function of both the magnetic coupling, from the exterior of the push-button to the plunger and the plunger's mass. Because of the decreased effective forces the entrance into the passageway must be correspondingly larger. The resulting compromises in construction are believed to leave such a switch vulnerable to premature closing due to the shock or vibrational forces.
A further limitation is found in the variations in the dimension of the passageway with respect to the contact as the plunger is withdrawn from the pool. To ensure good "making," wetted contacts are used. However, such wetted contacts tend to hold onto the liquid conductor. Thus, it is desired that the liquid conductor self-propel itself from the passageway. It is for this reason that the passageway is provided with converging walls. As the liquid conductor propels itself from the passageway it "necks down" and eventually breaks with the wetted contact. It is desirable that this breaking should be as soon as possible. As the plunger is withdrawn, however, the space between the contact wall and the plunger wall becomes increasingly larger, thereby providing a wider path for the flow of the liquid back to the pool. The withdrawing wall also increases the dimensions of the passageway at a time when it is desired to either keep such dimensions constant or, at best, narrower to encourage the self-removal of the liquid and the breaking of the liquid thread. The widening passageway encourages a thicker thread, thereby slowing down breaking off of the contact between the pool and the wetted contact.